It is difficult to guess in what order things happened without seeing it.

I guess petite as you are you are probably the straw that broke the donkey's back - I guess that donkey was already overloaded.
Either the cistern was faulty from new and the right combination of weight and temperature just finished it off or it was badly fitted or refitted if it had been removed for maintenance. Porcelain is like glass as a flaw can slowly spread over time while the item continues to perform it's function and then finally suddenly fail...

I have one vague suspicion - the cistern or tank would usually be fixed to the wall or at least tied there by pipes. In a toilet where the cistern is separate it wouldn't matter if the pan moved a little when someone sat down or if over time the floor settled a little but in a close coupled toilet and cistern it would matter a great deal as any movement in the pan would place stress on the cistern, pipes, fixings...
Porcelain is very good but doesn't take kindly to being stretched, bent or flexed...

Similar forces are at work with variations in temperature that cause expansion/contraction which is why fixing holes in porcelain tend to be oversized and mounting washers are used - to allow a certain amount of lateral movement...

Might have been defective with a small crack from the very beginning and over time it just got bigger and bigger until it just gave way and ........................ Happens